The present invention relates to a metering system and, more particularly, to a modular system for measuring electricity consumption by a plurality of loads.
Electric power is typically generated at a remote, central generating facility and transmitted to the consumer over a distribution grid. To reduce transportation losses, a step-up, sub-transmission transformer is used to increase the voltage and reduce the current for transmission over the transmission lines of the distribution grid. The actual transmission line voltage usually depends on the distance between the sub-transmission transformers and the consumers of the electricity but is commonly in the range of 2-35 kilo-volts (“kV”). Distribution substation transformers and distribution transformers of a utility's secondary power distribution system reduce the voltage from the transmission line level to a distribution voltage for delivery and use by industrial, commercial, and residential consumers. In the United States, for example, electric power is typically delivered to a facility as a 60 Hertz (Hz), alternating current (AC) voltage ranging from 120-660 volts (“V”), depending upon the use.
While the total power consumption of a building or other facility is monitored by the electric utility with a power meter located between the distribution transformer and the facility's power distribution panel, in many circumstances, particularly in business environments, it is desirable to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, machinery, etc. or to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility. These loads are typically connected to one or more of the branch circuits that extend from the power distribution panel and each may be supplied with single phase or multi-phase power. In addition, it is often desirable to monitor several parameters related to efficient electric power distribution and consumption, such as active power, the time rate of transferring or transforming energy; the apparent power, the product of the root mean square (RMS) voltage and current; and the reactive power, the product of the RMS voltage and the quadrature component of the current. Flexibility has favored adoption of digital power meters incorporating data processing systems that can monitor a plurality of circuits and calculate the desired output parameters.
As generated, the fundamental AC voltage and current of the U.S. power grid approximate in-phase, 60 Hertz (“Hz”) sine waves over time. The effective or true power of the analog sinusoidal voltage and current waveforms is the integral of the product of the instantaneous magnitudes of the voltage and current averaged over a time period, usually a cycle of the waveform:
                    P        =                              1            T                    ⁢                                    ∫              0              T                        ⁢                          (                                                v                  ⁡                                      (                    t                    )                                                  ⁢                                  i                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        (        1        )            
where: P=effective or true power (watts)                v(t)=instantaneous voltage at time t        i(t)=instantaneous current at time t        T=time period, typically a waveform cycle period        
Referring to FIG. 1, in a digital power meter 20 the effective power is typically approximated by averaging the sum of a plurality of products of the instantaneous amplitudes of the voltage and current that are obtained by sampling the voltage and current waveforms at periodic intervals for a period making up at least one cycle of a waveform:
                    P        ≅                              1            T                    ⁢                                    ∑                              k                =                1                                            k                =                                  T                                      Δ                    ⁢                                                                                  ⁢                    t                                                                        ⁢                                          v                ⁡                                  (                  k                  )                                            ⁢                              i                ⁡                                  (                  k                  )                                            ⁢              Δ              ⁢                                                          ⁢              t                                                          (        2        )            
where: P=effective power                v(k)=sample voltage for the k-th sample, for example voltage 24        i(k)=sample current for the k-th sample, for example current 26        Δt=sampling intervalA digital power meter 20 comprises, generally, at least one voltage transducer 22, at least one current transducer 28, voltage and current sampling units 30, 32 and a data processing unit 34 to control the sampling units, read the instantaneous magnitudes of the voltage and current, and calculate the power and other output parameters from a plurality of voltage and current sample values.        
The voltage transducer 22 is commonly a voltage divider network that is connected to the conductor in which the voltage will be measured. The exemplary power meter 20 includes three voltage transducers 22, 24, 26 each connected to a bus bar 36, 38, 40 in a power distribution panel 42. Each of the bus bars conducts a single phase of the three-phase power delivered to the power distribution panel from the supply 44, typically the distribution transformer supplying the facility. The power distribution panel provides a convenient location for connecting the voltage transducers because the voltage and phase is the same for all loads attached to a bus bar and interconnection of the transducer and the facility's wiring is facilitated by the wiring connections in the power distribution panel. However, the voltage transducer(s) can be interconnected anywhere in the wiring connecting the supply and the load, including connection at the terminals of a load, for example, terminals 46, 48, 50 of the exemplary 3-phase load 52 or the terminal of the single-phase load 54.
A current transducer 28 typically comprises a resistor network 56 and an associated current transformer 58 that, when installed, encircles the conductor in which current flow is to be measured. A current transformer comprises a secondary winding 60, typically, comprising multiple turns of conductive wire wrapped around the cross-section of a toroidal core 62. The conductor of the current to be measured is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer. The primary winding has N1 turns (typically, N1=1) and the secondary winding has N2 turns and, thus, the current transformer has a turns ratio (n) of N1/N2. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is connected to the resistor network. The magnitude of the primary current can be determined from the amplitude of the voltage at the output of the resistor network.
To measure the power consumed by a load, a current transformer is installed encircling each conductor conducting power to the load. For example, three current transformers are arranged to encircle three conductors 64, 66, 68 connecting the exemplary 3-phase load 52 to the supply 44 and a single current transformer 70 encircles a single conductor 72 connecting the exemplary single-phase load 54 to the supply. (Neutral conductors are not illustrated). Bowman et al., U.S. Pat. No. 6,937,003 B2, discloses a power monitoring system that includes a plurality of current transducers mounted on a common support that maintains a fixed spatial relationship between the current transformers and simplifies installation adjacent to the rows of circuit breakers 16 in the typical power distribution panel. Similarly current transformers 58, 74, 76 of the exemplary power meter system 20 and the associated resistor networks are mounted on a single support structure 78 and connected to the power meter by a ribbon cable 80. On the other hand, the current transducers, for example current transducer 70, may be individual units connected to the power meter by individual wires. A current transformer may have a single piece core or a split core to facilitate encirclement of connected wiring with the toroidal core of the transducer. The current transformer 70 is an example of a split core transformer that is commonly installed proximate a pre-wired load or in a pre-wired power distribution panel.
The digital power metering system includes pluralities of voltage and current transducers and multiplexers 82, 84 sequentially connect respective transducers to respective voltage and current sampling units 30, 32. The sampling units 30, 32 each comprise a sample and hold circuit that periodically holds the output voltage of the respective transducer constant and a quantizer that converts the analog output voltage of the transducer to a digital signal. In the sampling units, time, the independent variable of the sinusoidal waveform equation, is converted from a continuum to a plurality of discrete moments and the concurrent magnitudes of the voltage or current transducer signals are converted to discrete, binary values of finite precision. A clock 86, which may be included in the data processing unit 34, provides a time reference enabling the data processing unit to output at least one sampling signal 88 to trigger the sampling of the voltage and current by the respective sampling units 30, 32.
The outputs of the sampling units are read by the data processing unit 34 which, in a typical digital power meter, comprises at least one microprocessor or digital signal processor (DSP). The data processing unit reads and stores the digital values quantifying the magnitudes of the current and voltage samples and uses the values to calculate the current, voltage, power, and other electrical parameters that are output to a display 90 for immediate viewing or to a communications interface 92 enabling transmission to another data processing system, such as a building management computer, for remote display or further processing, for example formulating instructions to automated building equipment. The digital power meter also includes a memory 94 in which operating instructions for the data processing unit, current and voltage samples, and calculated output are stored.
In addition, accurate measurement of electric power requires compensation for error introduced by the transducers of the power meter. For example, the secondary current of a current transformer is ideally equal to the current in the conductor (the primary winding) divided by the number of turns in the secondary winding. However, magnetization of the core of the transformer produces ratio and phase errors which depend on the magnitude of the current being measured and the configuration of the particular transformer, including factors such as core material and turns ratio. Typically, error compensation factors are ascertained by experimentation with sample transformers of each production batch. Error compensation factors for correcting the calculated output of the meter are also stored in the memory 94 for use by the data processing unit 34.
While initial installation of the digital power meter 20 is simplified by the available choices of current transformers and the simple connections of the voltage and current transducers to the power meter, the power meter's integration makes field repairs, modifications and updating problematic. The power meter is tested and calibrated with a specific set of current and voltage transducers and modification of a meter or replacement of a failed transducer requires recalibration of the power meter. A field repairperson typically does not have the equipment necessary to recalibrate the power meter and store new error correction data or a revised transducer configuration in the power meter's memory. As a result, it may be necessary to install a new calibrated meter or accept inaccurate readings from a meter that has been altered by repair.
What is desired, therefore, is a power meter providing flexible construction, simplified installation and improved serviceability.